The present invention relates to a servo device for a multineedle sewing machine, and more particularly to a servo device for a multineedle sewing machine which includes two needle bars; driving and stop means for said needle bars; means for engaging and disengaging the two needle bars relative to the driving and stop means; a feed-length adjustment device associated with at least one feed dog for the transport of the material being sewn; an adjustable thread tensioning device for the needle threads; means for locking each needle bar selectively in its highest position; positioning drive means for the multineedle sewing machine; and means including a position indicator attached to an arm shaft of the multineedle sewing machine, for adjusting the speed of rotation and positioning the needle bars in predetermined positions.
A switching device for a multineedle sewing machine for optionally engaging and disengaging the needle bars is disclosed in Federal Republic of Germany Utility Model No. 83 35 949, expressly incorporated by reference herein. In that disclosure, the switching device makes it possible for each needle bar, by activation of an electromagnet in cooperation with a pivotally mounted lever mechanism, to be locked in its highest position. In this position, the needle bar is in its disengaged position, i.e., it now cannot be moved either up or down.
The above-mentioned arrangement for engagement or disengagement of the needle bars has the following disadvantages:
1. If the engagement or disengagement is to take place exactly at the top dead center of the needle bars, i.e., at their highest position during the course of their movement, then in the case of so-called corner seams, it will not be possible to terminate two parallel-extending seams precisely at the correct point, before the disconnecting of a needle bar, to provide the proper course of the seam. This problem is due to the relatively large distance between the top surface of the workpiece being sewn and the tips of the sewing needles. As a result, the correct course of the seam is not assured when making corner seams.
2. On the other hand, if the engagement or disengagement is not to take place at the top dead center of the needle bars, then if the seam terminates directly before the disengagement of a needle bar, the above-mentioned distance is decreased or becomes equal to zero, if the tips of the sewing needles contact the top surface of the workpiece, or pass through the workpiece. However, the needle bar which is to be disengaged can only be locked at the top dead center. Therefore, until the moment of disengagement, the part being sewn continues to be pushed forward by an amount which is referred to hereinbelow as a partial stitch length. To avoid damaging the part being sewn, the sewing machine operator must commence the aforementioned termination of the seam in advance, by an amount equal to this partial stitch length, if exact corner sewing is to be obtained. In actual sewing operations, this early termination of the seam, directly before the disengagement of a needle bar, cannot be achieved with the required precision. Thus, here also, the correct course of the seam is not assured when sewing corners.
Furthermore, the engagement and disengagement of the needle bars of a multineedle sewing machine, which may be provided by a ball-detent mechanism, is known, for example, from Federal Republic of Germany Pat. No. 955,023, expressly incorporated by reference herein. Since in this device too, the needle bars are locked at the top dead center, the aforementioned disadvantages are also present in this multineedle sewing machine.